The great Western creed of the Trinity and the Incarnation — 'Whosoever will be saved' — composed in Latin in the fifth or sixth century. It bears the name of Athanasius, the champion of Nicaea, but is not in fact his work.
Whosoever will be saved, before all things it is necessary that he hold the Catholic Faith. Which Faith except every one do keep whole and undefiled, without doubt he shall perish everlastingly. And the Catholic Faith is this: That we worship one God in Trinity, and Trinity in Unity; neither confounding the Persons, nor dividing the Substance. […] So the Father is God, the Son is God, and the Holy Ghost is God; and yet they are not three Gods, but one God. […] This is the Catholic Faith, which except a man believe faithfully, he cannot be saved.
The dated sources the library holds, oldest first — in their own words.
The Quicumque is composed in the Latin West — most probably in Gaul — in the fifth or sixth century, and not in the Greek East of Athanasius; its language and structure are thoroughly Western.
“…the opinion that St. Athanasius was not its author. His reasons may be reduced to the two following: firstly, no early writer of authority speaks of it as the work of this doctor; and secondly, its language and structure point to a Western, rather than to an Alexandrian, origin.”
Though it carries his name — for its ringing defence of the Trinity and Incarnation that Athanasius championed at Nicaea — the creed is not his. Its authorship has been ascribed variously to St. Hilary, St. Vincent of Lérins, Eusebius of Vercelli, and others.